A lithography system for the fabrication of microdevices, as illustrated in FIG. 1, generally comprises a light source 100, an illumination system 110, a reticle (sometimes called a mask or photomask) 130, a lens system 140, and a stage 145 to hold a semiconductor wafer 150. Light 102 of wavelength λ from the source 100 is directed by the illumination system 110 onto the reticle 130, and the lens 140 collects the light from the reticle and forms an image on the wafer 150. The wafer is coated with a photosensitive material, called a photoresist, and after exposure and suitable development, the pattern of the image is converted into a pattern in the photoresist, used to control subsequent manufacturing steps.
These imaging systems, although capable of achieving resolutions as small as a quarter to a third of the wavelength used for exposure, are still limited by diffraction phenomena and the ability of the lens to collect enough light emerging from the photomask. For state-of-the-art systems, λ=193 nm, and therefore, with conventional lens aperture (NA=0.93), the ultimate resolution that can be achieved is approximatelyL=0.25λ/NA=52 nm.
Optical systems capable of producing smaller features, usually through near-field or evanescent phenomena, are considered “super-resolution” patterning systems.
For the fabrication of smaller features using conventional lithographic processing techniques, a shorter wavelength must be used. Such solutions are possible, but have practical limitations. For shorter optical wavelengths, (λ<185 nm), oxygen in the air absorbs the photon energy, as does quartz, the normal lens fabrication material. Vacuum systems and reflective optics can be used, and have been demonstrated for systems using plasma induced extreme ultraviolet (EUV) radiation at λ=13 nm. However, no conventional light sources at 13 nm exist, and generating enough photons and fabricating masks and lenses using multilayer reflectors (which typically have 70% reflection efficiency) provides an economically unattractive solution.
Electron beams can also be used. 50 keV electrons have a λ=0.001 nm, certainly small enough to resolve small features, and electron beam imaging systems, operating again in a vacuum, have been constructed and used to fabricate features near atomic dimensions. However, unlike photons, electrons are charged, and in high enough densities, repel each other, causing image distortion. This has proven an insurmountable problem for electron projection lithography, and most electron beam systems are serial—exposing a single spot at a time with a single beam. Furthermore, with each electron accelerated to an energy of 50 keV, the dose needed to expose resist can be provided by only a few hundred electrons, making statistical shot noise a problem for exposure uniformity.
Smaller resolution is not the only problem facing contemporary IC patterning. The complex masks enabling sub-wavelength exposure can be very expensive, due to the complex processes needed to fabricate them and their generally low yield. Although ICs to be produced in large volume can absorb these costs, low volume ICs and prototypes (which are generally discarded, not sold) suffer from the costs and delays in reliably producing advanced photomasks. Such projects, which may see low or no return on the investment, would benefit from a fabrication technique that is both rapid and inexpensive.
To achieve fabrication of features smaller than 40 nm, especially when large numbers of features are desired simultaneously at dense pitches, as in a conventional IC, there is therefore a need for a direct-write, or “maskless” high resolution lithography system. The need for these is greatest for low volume and prototype applications.
Ideally, such a maskless system would be compatible with the conventional optical infrastructure currently deployed in IC fabs. Ideally, such a system could be used with throughputs comparable to conventional exposure tools. If feasible, the exposures should also be verified as they are being written, as there is no mask that can be inspected (as in “conventional” IC manufacturing) to guarantee that the pattern data has been accurately transcribed.
One proposed maskless system uses parallel electron beam writers, such as the MAPPER system proposed by MAPPER Lithography of the Netherlands. This is illustrated in FIG. 2. In this system, an electron beam 202 is emitted from an electron source 200, and electron steering electronics 210 divide this beam into as many as 13,000 parallel electron beam channels. These are guided by electron lenses 220 towards the wafer 250 to be exposed. At the wafer exposure occurs by raster scanning the beams using the final beam controllers 240. En route, each beam passes through an aperture 232 in an aperture plate 230, where individual signals from blanking electronics 264 modulate the individual electron beams. These blanking electronics 264 are directed using signals passed over optical fibers 262, in turn driven directly by a data processing system 260 using the IC layout data. The layout data directing the blankers is synchronized with the wafer stage position so that the exposure matches the correct dose for that particular wafer location.
This can work in principle, since the individual electron beams can provide the resolution, and the parallel channels provide the speed. In practice, this system is prone to jitter and thermal instability, and since the energy associated with each electron is so high, shot noise remains an issue. Special e-beam materials must also be used, and are not those currently used in a conventional optical lithography process, which leads to compatibility issues when inserted into a contemporary fab.
Optical maskless systems driven directly by layout data have also been proposed. Such a system is illustrated in FIG. 3. These systems are similar to conventional optical lithography systems, in that they also have light 302 emitted by a source 300, which is then shaped by an illumination system 310. The final imaging is also accomplished by a lens system 340, which forms an image of a “mask” 330 onto a wafer 350 mounted in a stage 345. However, here a conventional optical mask is replaced by a dynamic mask 330 comprising reflecting elements 332, and the light from the source is usually directed to the dynamic reflecting mask 330 using a beamsplitter 315 designed into the optical system. The reflecting system elements 332 of the dynamic mask 330 in turn comprise many individually driven MEMS devices, with individual elements driven with signals corresponding to pixels in the layout pattern. These MEMS devices can be an array of micromirrors, tilting at various angles to modulate the reflected light, or small elements that move in and out, sometimes called a piston mirror configuration, that modulates the phase of the reflected light. The settings of the individual pixels are driven by a data processing system 360 that sends signals derived from the IC layout through connectors 362 to circuitry 364 on the active mask 330, which sets the mirrors appropriately. The individual pixels of the micromirror array (typically comprising 1028×1028 micromirrors) or the piston array are usually 20× to 200× larger than the corresponding pixels on the wafer, and the lens system has components 320 that are designed to reduce the image appropriately.
A single array is typically far too small to represent the entire layout of an IC at once. Instead, the layout data must be split into tiles representing sub-sections of the layout that are passed in sequence to the array for exposure. The final image is then stitched together using multiple exposures of the pattern data onto the wafer 350, with the wafer stage 345 moving in a manner synchronized with the transmission of data to the dynamic reflecting array. A transmitting element, such as a liquid crystal light valve or array of thin film transistors can also be used to provide a similarly dynamic mask with either reflecting or transmitting pixels.
These systems have the advantage of using conventional optical or UV wavelengths, and therefore the infrastructure of optical materials and process knowledge and experience can be leveraged to make the adoption of this technology less disruptive. However, since conventional lens designs and materials are used, such systems are therefore subject to the same resolution limitations as conventional optical lithography.
There is therefore a need for an exposure system capable of writing extremely small, “super-resolution” patterns (that is, patterns with feature sizes smaller than the resolution obtainable by conventional optical lithography) that is also compatible with the process infrastructure currently in place in IC fabrication facilities.